1. Field of the Invention
The present invention relates to an apparatus and method for transmitting a data frame in a wireless local area network (WLAN) terminal, and more particularly, to an apparatus and method for transmitting a data frame in a WLAN terminal in which when a self-CTS (clear to send) or RTS/CTS (request to send/clear to send) frame which is a protection mode frame to prevent a collision of frames transmitted between IEEE 802.11b and IEEE 802.11g devices is transmitted, an overhead resulting from the self-CTS or RTS/CTS frame is reduced by increasing a frame duration value and then transmitting data frames of QHYR number, and the throughput of the 802.11g device is increased by determining the number of data frames to be transmitted depending on the current transmission rate while achieving fairness in wireless transmission.
2. Description of the Related Art
In general, IEEE 802.11 is a collection of standards for a wireless local area network (WLAN) developed by an IEEE working group and currently includes IEEE 802.11, IEEE 802.11a, IEEE 802.11b, and IEEE 802.11g. The four standards use a carrier sense multiple access/collision avoidance (CSMA/CA) protocol which is an Ethernet protocol to share a path. IEEE 802.11b provides a transmission rate of about 11 Mbps per second, whereas IEEE 802.11g, which is most currently approved, provides a transmission rate of maximum 54 Mbps in a relatively short distance. Since IEEE 802.11g operates at a band of 2.4 GHz like 802.11b, there is compatibility between the two standards.
An 802.11g protection mode is provided to protect a data frame transmitted between 802.11g equipment and 802.11b equipment which are compatible with each other. The 802.11g protection mode is classified into a request to send (RTS) method, a clear to send (CTS) method and a self-CTS method.
When an 802.11g wireless terminal transmits a data frame modulated by an orthogonal frequency division multiplexing (OFDM), the 802.11b equipment may not detect it and determine that a channel is not busy, and thus it may attempt to transmit a frame, thereby resulting in a collision with a data frame transmitted from the 802.11g wireless terminal. The 802.11g protection mode is used to protect a data frame from a possible collision. As the 802.11g protection mode, the RTS method, the CTS method, or the self-CTS method is usually used.
In the 802.11g protection mode, an RTS/CTS or self-CTS frame is modulated to be recognized by the 802.11b equipment and then transmitted before an actual data frame is transmitted, thereby protecting the 802.11g frame. At this time, the RTS/CTS or self-CTS frame has information about a time required to transmit the 802.11g frame and to receive an ACK (acknowledgment) frame which is recorded in its duration field.
The wireless equipment that receives the RTS/CTS or self-CTS frame does not transmit data during the time recorded in the duration field, and, thus, a wireless channel is guaranteed between the wireless equipment that transmits and receives data.
However, the following problems occur as the RTS/CTS or self-CTS frame is modulated to be recognized by the 802.11b wireless equipment and then transmitted before the 802.11g frame is transmitted as described above.
First, the RTS/CTS or self-CTS frame should be transmitted to prevent a collision of packets, and thus an overhead occurs which leads to reduction of a transmission rate (i.e., throughput).
In addition, when the 802.11g wireless terminal competes with the 802.11b wireless terminal, since a link speed of the 802.11g wireless terminal is higher than that of the 802.11b wireless terminal, the 802.11g wireless terminal occupies a channel during a relatively short time and thus undergoes relatively big reduction in transmission rate.